The invention is directed to a single mode optical waveguide fiber having a large effective area. The large effective area is achieved using a low refractive index core center region surrounded by an annular region of relatively high refractive index.
Recently the effort to provide very high performance waveguide fibers, i.e., waveguides suited to very high data rate telecommunication systems having large regenerator spacing, has focused on waveguide cores having compound refractive index profiles. Examples of such compound cores are found in U.S. Pat. No. 5,613,027, Bhagavatula (the ""027 patent).
In the ""027 patent a family of core refractive index profiles was disclosed which was characterized by a maximum relative refractive index, xcex94%, spaced apart from the centerline of the waveguide fiber. The family of profiles provided exceptional properties well suited to high performance telecommunication systems. In addition, it was noted that certain members of the core profile family were simple in design and thus easier to manufacture and more cost effective. An embodiment in the ""027 patent comprised a core having a central core region in which the refractive index was lower than that of the clad layer of the waveguide fiber.
Some recent work by Nouchi et al., ECOC 1996, Oslo and IWCS Proceedings, pages 939-945, 1996, examined this low refractive index central region embodiment and reported effective area in the range of about 60 xcexcm2 to 100 xcexcm2 and good bend resistance. However, the work does not address the question of optimum placement of the zero dispersion wavelength and cut off wavelength.
The invention disclosed and described in this specification does consider the fuller range of waveguide fiber properties while providing effective areas well in excess of 100 xcexcm2 and bend resistance comparable to that of standard step index single mode waveguide fiber.
The effective area, xcex94eff, is
Aeff=2xcfx80(∫E2 r dr)2/(∫E4 r dr),
where the integration limits are 0 to 8, and E is the electric field associated with the propagated light. An effective diameter, Deff, may be defined as,
Aeff=xcfx80(Deff/2)2.
The term xcex94%, represents a relative measure of refractive index defined by the equation,
xcex94%=100xc3x97(n12-n22)/2n12,
where n1 , is the maximum refractive index in a first region and n2is the refractive index in a reference region which is usually taken to be the cladding region.
The term refractive index profile or simply index profile is the relation between xcex94%, or refractive index, and radius over a selected portion of the core. The beginning and end point of the selected segment may be described by widths or by radii referenced to the waveguide fiber centerline.
The term alpha profile refers to a refractive index profile which follows the equation,
n(r)=n0(1xe2x88x92xcex94[r/a]xcex1)
where r is radius, xcex94 is defined above, a is the last point in the profile, r is chosen to be zero at the first point of the profile, and xcex1 is an exponent which defines the profile shape. Other index profiles include the shapes such as a step, a trapezoid, and a rounded step, in which the rounding may be due to dopant diffusion in regions of rapid refractive index change.
The bend resistance of a waveguide fiber is expressed as induced attenuation under prescribed test conditions. A bend test referenced herein is the pin array bend test which is used to compare relative resistance of waveguide fiber to bending. To perform this test, attenuation loss is measured for a waveguide fiber with essentially no induced bending loss. The waveguide fiber is then woven about the pin array and attenuation again measured. The loss induced by bending is the difference between the two measured attenuations. The pin array is a set of ten cylindrical pins arranged in a single row and held in a fixed vertical position on a flat surface. The pin spacing is 5 mm, center to center. The pin diameter is 0.67 mm. During testing, sufficient tension is applied to make the waveguide fiber conform to a portion of the pin surface.
A focused study of the profiles of U.S. Pat. No. 5,613,027, Bhagavatula, has resulted in the identification of a sub-set of the profiles of the ""027 patent which display exceptional properties well suited to high performance telecommunications systems. The sub-set of profiles which are the subject of this application are especially advantageous because they are among the simplest of the profiles of the ""027 patent.
A first aspect of the novel single mode waveguide refractive index profile comprises a core region surrounded by a clad layer. The core has two segments, a first circular segment centered on the long axis of the waveguide and an abutting annular segment. Each segment is characterized by an index profile, a relative index, xcex94%, and a radius or width. Throughout this document the relative index is defined in terms of the reference index, nc, which is the minimum index of the clad layer. The first segment has a relative index, xcex941 % in the range of about xe2x88x920.05% to xe2x88x920.60% and a radius in the range of about 1 xcexcm to 5 xcexcm. The second segment has a relative index in the range of about 0.5% to 1.6% and a width in the range of about 1 xcexcm to 20 xcexcm. This width is measure from r1 to the last point of the second segment.
The refractive index profiles of the first or second segment may have different shapes such as a step, a rounded step, an  -profile, a trapezoid, or a triangle. In general, for any combination of profiles, xcex94 %""s and radii can be found to provide the required fiber properties. Because the first segment has a negative refractive index relative to the reference index, nc, which is the minimum index of the clad layer, the first segment profiles will be inverted relative to the second segment profiles.
Embodiments of this first aspect which include a second segment profile of a step, a triangle, and a trapezoid are presented in detail below.
The waveguide properties provided by the novel refractive index profile are effective area, xcex94eff, greater than 100 xcexcm2, total dispersion slope in the range of about 0.07 to 0.1 ps/nm2-km, mode field diameter in the range of about 8 xcexcm to 10 xcexcm, and cut off wavelength in the range of 1500 nm to 2000 nm. This cut off is measured for waveguide fiber in the uncabled state. The cabling process typically reduces cut off wavelength by about 400 nm to 450 nm so that the novel waveguide is single mode in the wavelength range 1000 nm to 1600 nm. The waveguide could of course be tailored to have a different cut off wavelength.
For wavelength division multiplexing applications, especially in systems which use optical amplifiers, it is advantageous to place the zero dispersion wavelength outside the operating window in the range of about 1530 nm to 1560 nm. The novel index profile provides for such zero dispersion wavelengths as is seen in the data tables below.
In a second aspect of the invention an additional or third segment may be added to the profile to improve bending and mircobending performance of the waveguide without effecting the ability of the refractive index profile to meet the functional requirements noted above. The third segment has a negative relative index, xe2x88x92xcex943 %, in the range of about xe2x88x920.05% to xe2x88x920.6% and a width, w3, measured from the last point of the second segment to the last point of the third segment, in the range of about 1 xcexcm to 20 xcexcm. The ranges on the parameters of the first and second segments remain as set forth above. As before, the first or second segment may have different shapes such as a step, a rounded step, an xcex1-profile, a trapezoid, or a triangle.
Another embodiment of this second aspect includes a fourth ring of positive relative index, similar to the second ring and a fifth ring of negative relative index similar to the first or third ring.